The present invention relates to a method of making a cellular (e.g. honeycomb) seal as may be used, for example, in a turbine.
Honeycomb seals are used in multiple locations in various gas turbines. For example, such seals may be used against the rails on shrouded buckets as an abradable material. The temperatures encountered at these locations can be relatively high, including 870° C. or more. Unfortunately, even a honeycomb material made from an oxidation resistant alloy can experience oxidation and a shortening of useful life under these conditions. For this reason, advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-based superalloys for making honeycomb materials and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc. For example, Haynes 214® (provided by Haynes International of Kokomo, Ind.) is an oxidation-resistant alloy constructed from 75 Ni, 16 Cr, 4.5 Al, 3 Fe, 0.05 C, 0.01 Y, 0.5 Mn, 0.2 Si, 0.1 Zr, and 0.01 B (by weight percent). However, even when constructed from this material, the expected life of a honeycomb seal in stage 2 shrouds can be less than 20,000 hours.
Aluminum-containing coatings, particularly diffusion aluminide coatings, have found widespread use as environmental coatings on gas turbine engine components. During high temperature exposure in air, aluminum-containing coatings form a protective aluminum oxide (alumina) scale or layer that inhibits corrosion and oxidation of the coating and the underlying substrate. Diffusion coatings can be generally characterized as having an additive layer that primarily overlies the original surface of the coated substrate and a diffusion zone below the original surface. The additive layer of a diffusion aluminide coating contains the environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material (mainly β(NiAl) if the substrate is Ni-base). The diffusion zone comprises various intermetallic and metastable phases that form during the coating reaction as a result of compositional gradients and changes in elemental solubility in the local region of the substrate.
Diffusion aluminide coatings are generally formed by depositing and diffusing aluminum into the surface of a component at temperatures at or above about 760° C. Notable processes include pack cementation and vapor phase aluminiding (VPA) techniques, and diffusing aluminum deposited by chemical vapor deposition (CVD), slurry coating, or another deposition process. Aluminum deposited by slurry coating is typically diffused without an activator in contrast to the other methods, relying instead on melting and subsequent diffusion of the deposited aluminum.
The processing temperature and whether an activator is used will influence whether a diffusion coating is categorized as an outward-type or inward-type. Outward-type coatings are formed as a result of using higher temperatures (e.g., at or above the solution temperature of the alloy being coated) and lower amounts of activator as compared to inward-type coatings. In the case of a nickel-based substrate, such conditions promote the outward diffusion of nickel from the substrate into the deposited aluminum layer to form the additive layer, and also reduce the inward diffusion of aluminum from the deposited aluminum layer into the substrate, resulting in a relatively thick additive layer above the original surface of the substrate. Conversely, lower processing temperatures and larger amounts of activator reduce the outward diffusion of nickel from the substrate into the deposited aluminum layer and promote the inward diffusion of aluminum from the deposited aluminum layer into the substrate, yielding an inward-type diffusion coating characterized by an additive layer that extends below the original surface of the substrate.
The choice of donor material influences whether an outward or inward-type diffusion coating can be produced since aluminum alloys such as CrAl, CoAl, FeAl, TiAl, etc., have higher melting temperatures than unalloyed aluminum and, therefore, can be used with the higher processing temperatures used to form outward-type coatings. Though both outward and inward-type diffusion aluminide coatings are successfully used, outward-type diffusion aluminide coatings typically have a more ductile and stable nickel aluminide intermetallic phase and exhibit better oxidation and low cycle fatigue (LCF) properties as compared to inward-type diffusion aluminide coatings.
Slurries used to form diffusion aluminide coatings are typically aluminum-rich, containing only an unalloyed aluminum powder in an inorganic binder. The slurry is directly applied to surfaces to be aluminized, and aluminiding occurs as a result of heating the component in a non-oxidizing atmosphere or vacuum to a temperature above about 760° C., which is maintained for a duration sufficient to melt the aluminum powder and diffuse the molten aluminum into the surface. The thickness of a diffusion aluminide coating produced by a slurry method is typically proportional to the amount of the slurry applied to the surface, and as such, the amount of slurry applied must be very carefully controlled.
The difficulty of consistently producing diffusion aluminide coatings of uniform thickness has discouraged the use of slurry processes on components that require a very uniform diffusion coating and/or have complicated geometries. As a result, though capable of forming diffusion aluminide coatings on internal and external surfaces, slurry coating processes have been typically employed to coat limited, noncritical regions of gas turbine engines. Another limitation of slurry coating processes is that, because of the use of unalloyed aluminum, they are typically performed at relatively low temperatures (e.g., below 980° C.), and are therefore limited to producing an inward-type coating with high aluminum content.
A method and composition for coating honeycomb seals and, more specifically, a method and slurry for applying an aluminide coating onto honeycomb seals is described in US2011/0074113 to Cavanaugh et al. The method includes preparing a slurry of a powder containing a metallic aluminum alloy having a melting temperature higher than aluminum, an activator capable of forming a reactive halide vapor with the metallic aluminum, and a binder containing an organic polymer. The slurry is applied to surfaces of the honeycomb seal, which is then heated to remove or burn off the binder, vaporize and react the activator with the metallic aluminum to form the halide vapor, react the halide vapor at the substrate surfaces to deposit aluminum on the surfaces of the seal, and diffuse the deposited aluminum into the surfaces to form a diffusion aluminide coating. While this process is very useful for forming an aluminide coating on honeycomb materials attached to superalloy substrates, various issues have been observed, including entrapment of residue from the aluminiding process in the cells, and migration of braze materials used to attach the cellular seal to the seal substrate within the cells during aluminiding where they may form undesirable compounds. Therefore, an improved method for making aluminized cellular seals is very desirable.